As well as converting incoming solar radiation into electricity using PV cells, energy from the sun can also be harnessed to provide domestic hot water. These systems do not generally provide space heating, and are described as 'solar thermal' systems. They are among the most cost-effective renewable energy systems that can be installed on dwellings in urban or rural environments. In a typical system, a heat transfer medium (generally a water or antifreeze mixture) travels through a series of heat conducting tubes known as a heat collector. During its circulation through the tubes, the fluid picks up heat which is then transferred to the domestic hot water supply as it passes through a coil in an appropriate storage cylinder.